Genus: Ipomovirus


Genus: Ipomovirus

Distinguishing features

Ipomoviruses are distinguished from members of other genera by their mode of transmission being by whiteflies, and by their separate branching on phylogenetic analyses.

 Virion

Morphology

Virions are flexuous filaments 800–950 nm long.

Physicochemical and physical properties

Virion Sedimentation coefficient S20,w is 155S for sweet potato mild mottle virus (SPMMV).

Nucleic acid

Virions contain a single molecule of linear, positive-sense ssRNA of about 9.7 kb; virions contain 5% RNA by weight.

Proteins

The viral coat (capsid) protein (CP) is a single polypeptide of 302–378 aa (35–41 kDa). 

Genome organization and replication

Ipomoviruses exhibit unusual structural variability. The structure and organization of the SPMMV genome is similar to that of members of the genus Potyvirus (Figure 2.Potyviridae), but some motifs of HC-Pro and CP characteristic of members of the genus Potyvirus are incomplete or missing, which may account for its vector relations. The unusually large P1 protein (83 kDa) of SPMMV contains no obvious AlkB domain and hence differs from that found in members of the genus Brambyvirus. Cucumber vein yellowing virus (CVYV) and squash vein yellowing virus (SqVYV) differ from SPMMV by encoding two P1-like serine proteases (P1a and P1b) but no HC-Pro. P1b functions as a suppressor of RNA silencing (Li et al., 2008). Cassava brown streak virus (CBSV) differs from SPMMV by having no HC-Pro and differs from CVYV and SqVYV by having only P1b which suppresses silencing (Figure 1.Ipomovirus). Additionally, cassava brown streak virus (CBSV) contains a Maf/HAM1-like sequence recombined into the NIb/CP junction, which can accommodate heterologous genes in engineered infectious potyvirus clones. Homology of HAM1h with cellular Maf/HAM1 NTP pyrophosphatases suggests that HAMh1 might intercept non-canonical NTPs to reduce mutation rates of viral RNA.

Figure 1.Ipomovirus. Genomic maps of the ipomoviruses sweet potato mild mottle virus (SPMMV), squash vein yellowing virus (SqVYV) and cassava brown streak virus (CBSV). The ssRNA genome is represented by a line and the polyprotein ORF by an open box. Conventions are as for the potyvirus genome organization map (Figure 2.Potyviridae). Activities of most mature proteins are postulated by analogy with genus Potyvirus. CVYV and SqVYV contain two P1-like serine proteases (P1a and P1b), of which P1b functions as a suppressor of RNA silencing. CBSV also contain P1b which suppresses silencing and, additionally, carries a Maf/HAM1-like sequence recombined into the NIb/CP junction. HAMh1 might intercept non-canonical NTPs to reduce mutation rates of viral RNA.

Antigenicity

Moderately immunogenic. No serological relationships with other members of the family Potyviridae have been found.

Biology

Host range

The natural host range of SPMMV is wide, with more than nine families susceptible, whereas the host range of CBSV, CVYV and SqVYV is less known apart from the hosts which they have been found to infect in the field (Winter et al., 2010).

Transmission

CBSV, CVYV and SqVYV are transmitted by the whitefly Bemisia tabaci in a non-persistent manner. B. tabaci may also be the vector of SPMMV, but this is not fully confirmed. All ipomoviruses are transmissible experimentally by mechanical inoculation and by grafting.

Species demarcation criteria

See discussion under family description.

Member species

SpeciesVirus name(s)Exemplar isolateExemplar accession numberExemplar RefSeq numberAvailable sequenceOther isolatesOther isolate accession numbersVirus abbreviationIsolate abbreviation
Cassava brown streak viruscassava brown streak virusKOR6GU563327NC_012698Complete genomeCBSV
Coccinia mottle virusCoccinia mottle virusSu12-25KU935732NC_030840Complete genomeCocMoV
Cucumber vein yellowing viruscucumber vein yellowing virusALM32AY578085NC_006941Complete genomeCVYV
Squash vein yellowing virussquash vein yellowing virusFLEU259611NC_010521Complete genomeSqVYV
Sweet potato mild mottle virussweet potato mild mottle virusEAZ73124NC_003797Complete coding genomeSPMMV
Tomato mild mottle virustomato mild mottle virusEthiopiaHE600072Complete genomeTMMoV
Tomato mild mottle viruseggplant mild leaf mottle virusIsraelHQ840786EMLMV
Ugandan cassava brown streak virusUgandan cassava brown streak virusUGFJ185044Complete genomeUCBSV

Virus names, the choice of exemplar isolates, and virus abbreviations, are not official ICTV designations.

Derivation of names

Ipomovirus: from Ipomea and mosaic, the symptoms induced on the host of members of the type species Sweet potato mild mottle virus.